1. Field of the Invention
This invention pertains to an improved multilayer printed wiring board, which is used for mounting and connecting electrical or electronics components, and a method of making the multilayer printed wiring board.
2. Description of the Prior Art
Multilayer printed wiring boards have generally been used in order to achieve high density packagings of electrical or electronics wirings. A typical conventional multilayer printed wiring board assembly employs a plurality of base board in a laminated form. Each of the base board has thereon a printed wiring of a predetermined pattern. Between two adjoining base boards with respective printed wirings are interposed adhesive insulation material (prepreg). Layers of base boards, printed wirings and adhesive insulation material are put together by being machine-pressed so as to adhere to one another to form a printed wiring board assembly. Through-holes are made in the board assembly at predetermined positions and the holes are copper-plated so that the individual layers of the printed wirings may be electrically interconnected.
When a press is used to fablicate a multilayer printed wiring board assembly as described above, the base boards are liable to shrink and warp because the boards are subjected to a high temperature during each pressing process. In order to prevent the boards from warping, a work margin of at least 60 mm must be added to each side of the boards. Such work margins are, however, nothing but substantial waste areas of the boards. Furthermore, the required number of pressing steps increases with the number of layers. For example, a six-layer board assembly would require two pressing steps and an eight-layer board assembly three pressing steps. Since the base boards are liable to shrink in each pressing step, as the number of layers increases the dimensional accuracy of the fablicated board assembly tends to be degraded. As a consequence, a limitation must exist in the number of layers which can practically be used for a board assembly.
In addition, the cost saving in such a conventional method of fablication is not an easy task because the press is expensive, a number of manufacturing steps, including preparatory steps, are required before the pressing step, and a substantial space for the press in addition to the work area has to be maintained.
The method of making a multilayer printed wiring board as described below has so far been proposed in order to solve the above described problems. Referring to FIG. 3, a base board 11 and a first layer printed wiring 12 are made from a copper-clad board in a conventional manner. Epoxy resin is applied to cover the first layer wiring 12 to form a first layer of epoxy resin 13 by means, for example, of a silk screening printing. Then, copper plating is applied onto the epoxy resin 13 to make a copper foil layer 14, which is subsequently made into a second layer printed wiring. Next, though not shown in the figure, epoxy resin is again applied to cover the second layer printed wiring in the same manner as described above. The same step is repeated for a desired number of layers. By making through-holes at predetermined positions of the board before the last copper plating is performed, indivisual layers of the printed wirings are electrically interconnected and the multilayer printed wiring board is completed.
The preceding manufacturing method of a multilayer printed wiring board requires neither pressing steps nor a spacious work area including the space required for a press. The work margins of the board, which is a waste, can be minimized and the number of layers can easily be increased as required. Thus, a substantial cost saving can be achieved.
However, a disadvantage still pertains to this manufacturing method. Namely, the bond between the layer of epoxy resin 13 and the copper foil layer 14 is not necessarily satisfactory and, therefore, the printed wiring is liable to peel off the epoxy resin.
Referring to FIG. 4, one idea to prevent such a peel-off problem of the wiring is that the surface of the epoxy resin layer 13 is finished rough so as to have numerous fine scratches 13a by means of brushing or sandblasting before the surface is copper-plated. However, because each of the fine scratches 13a tapers off toward its bottom as shown in FIG. 4, the bonding strength between the epoxy resin 13 and the copper foil layer 14 can not sufficiently be enhanced. A part of the plated copper can separate from the epoxy resin 13 also because of the pressure which may build up in the epoxy resin 13 when the moisture contained therein evaporates.
An idea of applying an electrically conductive paste onto the epoxy resin in order to make a wiring layer, instead of plating with copper, would lack practicality because such conductive paste would not provide a sufficient electrical conductivity and, therefore, problems tend to entail.